1. Field of the Invention
The present invention relates to the field of electrostatographic recording, such as electrophotography and electrography. More particularly, it relates to improvements in apparatus and methods for monitoring certain properties which characterize the electroscopic toner particles used in electrostatographic recording devices to develop electrostatic charge patterns and images.
2. Discussion of the Prior Art
It is well known that the quality of images produced by the electrostatographic image recording process is controllable by varying certain process parameters. These parameters include, for example, the primary charging voltage to which the image-recording element is initially charged; the amount of exposure received by the recording element for the purpose of imagewise dissipating the initial charge to form a developable charge pattern; the bias voltage applied to a development electrode during the development of the charge image; and the concentration of electroscopic toner in a development mixture of toner and carrier particles. Other important process parameters are the electrostatic charge-to-mass ratio of the toner particles comprising the developer, and the rate at which toner particles are accepted by the electrostatic charge-bearing member to effect development thereof. The latter parameter is known as the mass deposition rate or, more simply, the development rate. Many factors influence the charge-to-mass ratio and development rate of toner particles. These factors include relative humidity, toner concentration, chemical contamination, and developer mixture aging. With respect to the latter, all developer mixtures contain toner particles having diameters which vary within a specified range. For a variety of reasons, a charge image will more readily accept larger toner particles than smaller ones. Thus, with continued use of the same developer mass, there will be a gradual decrease in the average toner particle size in the developer mass. This, in turn, gives rise to an increase in the charge-to-mass ratio of the developer, and a decrease in the development rate.
Increases in the developer's charge-to-mass ratio usually result in a decrease in density of a developed image since fewer toner particles will be required to fully develop and thereby neutralize the charge image. On the other hand, decreases in the charge-to-mass ratio gives rise to over-development of the charge image since more particles are required for charge neutralization. Similarly, a decrease in toner's development rate will often give rise to a decrease in image density, and vice versa. Accordingly, it is desirable to monitor the charge-to-mass ratio and development rate of a toner so that other process parameters can be adjusted to compensate for changes in these paramenters. Feedback control can be used to adjust any of the aforementioned parameters to adjust image quality in response to changes in the charge-to-mass ratio and/or development rate.
In the commonly assigned U.S. Pat. No. 5,006,897 to D. S. Rimai et al., there is disclosed an apparatus for monitoring the development rate and charge-to-mass ratio of a moving mass of toner particles which is being applied to a charge image to effect development. Such apparatus includes a piezoelectric crystal having a planar electrode disposed on one surface thereof. The crystal and its associated electrode are positioned in the toner mass, and the electrode is electrically biased to attract toner to its outer surface. The mass of toner attracted to the electrode depends upon the bias voltage on the electrode and the charge on the toner. The greater the charge on the toner, the smaller the amount toner required to neutralize it. By energizing the crystal and measuring the shift in frequency caused by the deposition of toner on the biased electrode, a determination is made of the toner mass deposited on the electrode. The amount of charge on the toner is determined by a transient current measuring scheme in which the time interval required for the charge on the electrode to decay to a predetermined level is measured. The measured toner charge (as reflected by the measured charge decay time) is divided by the measured toner mass (as reflected by the measured frequency of oscillation of the crystal after the charge decay time) to produce a signal proportional to the charge-to-mass ratio. Moreover, by dividing the measured toner mass by the measured charge decay time, the toner mass deposition rate (i.e. the development rate) is determinable.
While the apparatus disclosed in the above-noted patent is useful in determining both charge-to-mass and development rate parameters, it is subject to certain limitations. For example, if either the toner or carrier particles comprising the developer are electrically conductive in nature, a DC signal may mask the transient current signal, making it difficult to detect. Also, the combined transient current and mass measurements require several variables to be determined simultaneously, including the toner mass deposited, the time needed to deposit that toner and the transient current. Finally, since the development rate varies as a function of the difference in bias potential between the toner applicator and the crystal, the transient current measurement only gives the average development rate over the charge decay time.